1. Field of the Invention
The present invention relates to a nonlinear filter to be used in radio communication devices such as mobile phones and wireless LAN devices.
2. Description of the Background Art
In general, a transmission circuit performs modulation by varying the amplitude and the phase of a carrier wave in accordance with an input signal, thereby generating a transmission signal. When variations of the transmission signal are observed on an IQ plane, it may be observed, depending on the modulation method, that the transmission signal passes near the origin of the IQ plane when transitioning from one symbol to another symbol. In such a case, the frequency band of the transmission signal is wide with respect to both the amplitude and phase components thereof. This causes a problem of, for example, interference with adjacent frequency bands.
In order to avoid such a problem, there is a known conventional technique called hole blowing. FIG. 17 shows a configuration of a nonlinear filter 1701 that performs a hole blowing process disclosed by Patent Document 1. The nonlinear filter 1701 includes a pulse shaping unit 1702, a delay unit 1703, a determination unit 1704, and a pulse generator 1706. An I signal (in-phase signal) and a Q signal (quadrature-phase signal) are inputted into the nonlinear filter 1701. The determination unit 1704 monitors variations of the I and Q signals on the IQ plane. When determining that the distances of the I and Q signals from the origin of the IQ plane are less than a predetermined value, the determination unit 1704 outputs an instruction signal to the pulse generator 1706 so as to instruct the pulse generator 1706 to output pulses. Based on the instruction signal, the pulse generator 1706 calculates an appropriate scale for the I signal and an appropriate scale for the Q signal, and outputs pulses of these scales. The respective pulses are added by the adder 1707 to the I and Q signals whose timings have been compensated for by the delay unit 1703, and the resultant I and Q signals are outputted as outputs from the nonlinear filter 1701. The outputted I and Q signals are, as compared to those having just been inputted into the nonlinear filter 1701, corrected by the added pulses so as to be directed away from the origin on the IQ plane. Note that the pulse shaping unit 1702 is provided at the input or the output of the nonlinear filter 1701, and performs pulse shaping.
FIG. 18 shows a configuration of a transmission circuit 1801 that includes the nonlinear filter 1701 as described above. The transmission circuit 1801 includes input terminals 1802 and 1803, the nonlinear filter 1701, a coordinate system converter 1804, LPFs (low-pass filters) 1805 and 1806, an angle modulator 1807, an amplitude modulator 1808, and an output terminal 1809.
I and Q signals are inputted into the nonlinear filter 1701 via the input terminals 1802 and 1803, respectively. The nonlinear filter 1701 performs the above-described process on the I and Q signals, and outputs the resultant I and Q signals. Based on the I and Q signals on which the hole blowing process has been performed, the coordinate system converter 1804 generates and outputs an amplitude signal and a phase signal. The amplitude signal is inputted into the amplitude modulator 1806 after being band-limited by the LPF 1805. The phase signal is inputted into the angle modulator 1807 after being band-limited by the LPF 1806. The angle modulator 1807 performs angle modulation based on the phase signal, thereby generating an angle-modulated signal. The angle-modulated signal is inputted into the amplitude modulator 1808. Based on the inputted amplitude signal, the amplitude modulator 1808 performs amplitude modulation on the angle-modulated signal to generate a transmission signal, and outputs the transmission signal via the output terminal 1809.
Owing to the above configuration, the transmission signal on the IQ plane is prevented from passing through a predetermined range from the origin. In this manner, the frequency band of the transmission signal can be suppressed from being widened.